31° South: phenotypic flexibility in adaptive thermogenesis among conspecific populations of an arid-endemic bird - from organismal to cellular level

Abstract

18In north-temperate small passerines, overwinter survival is associated with a reversibly increased 19 maximum cold-induced metabolism (M sum ). This strategy may incur increased energy 20 consumption. Therefore, species inhabiting ecosystems characterized by cold winters and low 21 productivity (i.e., low available energy) may be precluded from displaying an increase in 22 Dawson 1989) (Swanson and Garland 2009). An alternative explanation is offered by the climate 52 variability hypothesis (Janzen 1967), (Bozinovic and Naya 2014), which posits that broader 53 climatic fluctuations results in wider flexibility in thermal tolerance as a means to cope with the 54 fluctuating environmental conditions. 55 Several mechanisms, from whole-organism level down to the biochemical level, have been 56 proposed to explain the high cold-induced M sum : i) increase in body condition assessed as body 57 tarsus-length (calliper, d = 0.01 mm). Thus, M b-scaled = M i x [L 0 /L i ] bSMA , where M i and L i are the 127 body mass and tarsus-length of individual i, respectively; L 0 is the tarsus-length arithmetic mean 128for the study populations to which index is standardized; b SMA is the scaling exponent estimated 129 by the standardized major axis regression of mass-length. 130We checked fat accumulation at the furcular depression and abdomen in all 85 adult birds, and 131 quantified it using a scale that ranges from zero (no fat) to eight (flight muscles not visible with 132

31° South: The physiology of adaptation to arid conditions in a passerine bird

“…These short‐term changes in BMR are thought to arise from adjustments in the mass of metabolically active organs: mass of thermogenic muscles in winter (Zheng, Liu, & Swanson, ), and/or mass of reproductive organs in spring/summer of reproductive organs (Vézina, Salvante, & Williams, ). Although the striking increase in BMR during summer in Coastal and Central populations cannot be explained by hypertrophy of thermogenic muscles (Ribeiro, Prats, et al, ) we cannot rule out the possible effect of metabolic costs of maintaining reproductive organs. However, we find it unlikely, because the breeding season of subspecies cinerea (Coastal morph showing increase in summer BMR) typically occurs between September–October and we collected data from 11–18 November 2015.…”

“…Being aware that footprints of selection associated with spatial climatic variation do not necessarily imply causality, it is compelling that several of the putative adaptive SNPs (Table ) localize in genes encoding machinery responsible for skeletal fibre twitching (myosin, troponin and calcium channel) as this is the principal mechanism that birds use as a heat source in response to cold stress (shivering thermogenesis; Hohtola, ). This finding is simultaneously surprising and fascinating, because Inland birds did not increase winter thermogenic capacity through shivering (Ribeiro, Prats, et al, ; measured as whole organism O 2 consumption/CO 2 production under cold conditions), suggesting that natural selection may be favouring genotypes that have a high efficiency of fibre twitching without altering O 2 /CO 2 rates. Furthermore, the potential role of 5′AMP activated kinase in allowing local adaptation is noticeable.…”

“…Metabolic expansibility reflects the capacity of an organism to rapidly adjust its metabolism to meet the energetic challenge of thermoregulation under cold conditions (measured as the ratio of maximum thermogenic capacity and basal metabolism). Given the absence of regional differences in maximum thermogenic capacity (Ribeiro, Prats, et al, ), the wider metabolic expansibility observed in populations experiencing lower winter temperatures (Central and Inland populations; mean minimum temperature in winter <2°C) inescapably stems from the lower BMR. Thus, as opposed to northern‐temperate climates, where highly variable climates select for higher maximum thermogenic capacity (Swanson, Zhang, & King, ), subtropical and highly variable climates should select for lower BMR.…”

“…Metabolic expansibility was estimated as the ratio between M sum (maximum metabolic capacity) and BMR obtained for the same individuals: ME = M sum /BMR. We used M sum values (maximum thermogenic capacity, obtained as maximum VCO 2 at low helix temperatures) from Ribeiro, Prats, Pattinson, Gilbert, and Smit () and calculated ME for 61 adult scrub‐robins: Coastal = 21, Central = 18 and Inland = 22. Briefly, as reported in Ribeiro, Prats, et al (), M sum was measured as the bird's O 2 consumption and CO 2 production while exposed to a HelOx atmosphere (79% helium + 21% oxygen).…”

“…We used M sum values (maximum thermogenic capacity, obtained as maximum VCO 2 at low helix temperatures) from Ribeiro, Prats, Pattinson, Gilbert, and Smit () and calculated ME for 61 adult scrub‐robins: Coastal = 21, Central = 18 and Inland = 22. Briefly, as reported in Ribeiro, Prats, et al (), M sum was measured as the bird's O 2 consumption and CO 2 production while exposed to a HelOx atmosphere (79% helium + 21% oxygen). The birds were exposed to a sliding cold exposure protocol by reducing HelOx temperature by 3°C every 10 min.…”